1. Technical Field
The present invention relates generally to material analysis, and more particularly, to the field of impedance spectroscopy, and the determination of engineering properties of a material such as density and moisture content from the response to electromagnetic probing in a defined frequency spectrum.
2. Related Art
Determination of engineering properties of materials such as density and moisture content is oftentimes desired. The engineering properties desired vary depending on application. For purposes of this application, one example application is compacting of engineering materials such as asphalt concrete or soil, which may be used in paving, building foundations, or the like. In this application, the degree of compaction is regarded as critical to the long-term durability of such systems. Under-compaction will result in poor strength and eventual settling that can cause cracking. Over-compaction results in poor performance due to the limited ability to absorb loads or handle moisture absorption.
The Proctor test (ASTM D698 and ASTM D1557) is used in the laboratory to determine the optimum moisture content for compaction and the maximum achievable compaction for a given amount of compaction energy. Field material compaction to achieve best engineering properties is specified to be at least 95% of the applicable Proctor test. The Sand Cone Test (ASTM D1556) is a known field test that can measure material density directly, but conducting the test requires considerable time and operator skill to produce accurate results. This test also requires digging a hole in the material that must later be repaired.
Several indirect methods exist that attempt to relate a measurable property of the material, such as resistance to penetration, to the in-place compaction. Such devices are known to use nuclear methods, mechanical penetrometers (both manual and electronic), and electrical impedance methods to measure a property of the material that can be related to density. Unfortunately, conventional devices do not adequately measure moisture content in a material, which is a highly desirable parameter. In addition, many of the indirect devices suffer from a number of deficiencies such as requirements for special storage, handling, and training. These deficiencies may be the result of use of nuclear sources, long measurement times, operator and material interface induced inaccuracies, the need for penetrating probes that must be carefully installed, and/or the inability to provide accurate measurements over the range of materials typically encountered in engineering practice. For example, material type, gradation, moisture content, and conductivity are known to affect prior art devices.
The dielectric permittivity of a porous mixture undergoing a compaction process increases with increasing density. This results from the displacement of air (dielectric constant=1) by solid materials (dielectric constant=3–5) and water (dielectric constant=80) in any volume of the material. It is further known that the permittivity of composite dielectric materials includes three components: a real part and an imaginary part, the latter of which includes a conductivity part and a dielectric loss part. The real part of the permittivity is related to energy storage and is commonly referred to as the dielectric constant. It is known in the art that the real part of the permittivity at certain frequencies in the electromagnetic spectrum is related to the density of the material. The imaginary part of the permittivity is related to energy loss and includes, as noted above, a conductivity part and a dielectric loss part. The conductivity part is related to ohmic conduction due to free ions, and the dielectric loss part is due to polarization losses from molecular, atomic, and interfacial dipole effects. The presence and amount of the three permittivity components is a function of the chemical and geometrical constituency of the material.
A number of approaches exist to measure one or more of the permittivity components of a material in order to physically determine properties of the material, and in particular, density and/or moisture content. One approach is disclosed by Blackwell in U.S. Pat. No. 3,784,905. The device of Blackwell has many disadvantages. For example, in order to obtain a reading, the Blackwell device must be moved at extremely slow speeds across the material being tested and, accordingly, requires an extended time period to provide a determination. The Blackwell device, due to its excessive weight, also requires a large sled frame (contact area) to be dragged across the pavement surface. Another disadvantage is limited adjustability of the depth of measurement of the device caused by the given set of electrodes only being able to vary the depth of measurement by changing the height of the electrodes. In addition, this device measures only the real component of the asphalt permittivity at a single frequency. As a result, it is not possible to determine whether conductivity or moisture has affected the reading. Further, the frequency employed by the Blackwell device is in a range where surface polarization effects resulting from surface conductive water will make the reading inaccurate.
In another apparatus taught by Regimand, U.S. Pat. No. 4,766,319, a nuclear source is used to determine density of pavement material. While the nuclear approach is considered by many to be technically adequate, the device has a variety of practical drawbacks. For instance, the device requires a licensed operator and a radiation shield, e.g., a lead enclosure. Furthermore, the device is non-adjustable for area, time-consuming in use, and heavy. In addition, storage, use, and disposal are strictly regulated and pose users with significant logistical and monetary expense. Recent concerns for homeland security have resulted in initiatives to eliminate devices that could be used by terrorists.
Another approach is taught by Siddiqui, et al. in U.S. Pat. No. 6,215,317. This patent describes a method and apparatus that uses time domain reflectometry (TDR) to determine the dielectric permittivity of compacted material. A number of practical disadvantages exist with the Siddiqui device. First, the device requires a penetrating probe to be driven into the material. The act of driving a probe into the material causes the density to change in the vicinity of the probe, causing errors in measurement. Another disadvantage of this device is the need to accomplish a single point field Proctor test in order to separate the effect of material moisture on the dielectric response from that of the material density. This results in an overall time to make a measurement of 10–15 minutes. Such measurement results in significant additional time on the job site such as a city street on which traffic must be stopped while repairs are being made to, for example, buried utility company equipment. A third deficiency with the Siddiqui device is their use of non-insulated probes to make the measurement. For materials that may have high conductivity, such as engineering materials, significant attenuation and consequent loss in resolution and accuracy can result.
Another known approach operates by determination of complex permittivity, and is taught by Sovik et al. in U.S. Pat. No. 6,414,497, which is assigned to TransTech Systems, the assignee of the present invention. The Sovik device operates by transmitting electromagnetic energy at a single frequency into the material via an arrangement of electrodes of a sensor. The material being measured becomes the dielectric of a capacitor formed by the electrodes (sensor elements). By measurement of the total permittivity and suitable calculations, the dielectric constant of the material, and hence the density may be determined using a single variant linear regression: In addition, the loss tangent of the total impedance, calculated as the ratio of the imaginary part of the permittivity to the real part, is used by the Sovik device to indicate the presence of moisture on a top surface of the material that may affect the measurement. The Sovik device makes a first order correction for this moisture, but is incapable of determining the bulk moisture content in the material. Further this correction is susceptible to error caused by a variable and unknown conductivity in the surface water. Unfortunately, many materials used for engineering purposes, such as soil, typically contain 6–9% water by weight. Additionally, conductivity as high as 10 mS/cm may be present in the form of dissolved salts (such as NaCl) in these materials. The electromagnetic response of dielectric materials containing water is such that the effects of water, conductivity, and particle geometry and the effects of density on the dielectric response cannot be separated using measurements made at a single electromagnetic frequency. Additionally, the forward mathematical model suggested in Sovik to relate the impedance to the density is based upon a presumed form that can be described in terms of a passive electrical equivalent circuit. Unfortunately, for complex materials such as soil, no adequate theoretical models exist to explain the complex interaction between the soil surface, water, and dissolved ions.
Another device, invented by Dr. Max Hilhorst (PhD thesis, “Dielectric Characterization of Soil,” 1998), measures complex impedance at a single frequency to determine the moisture content and conductivity of soil. Devices that practice the teachings of Dr. Hilhorst are primarily applied to the determination of the moisture content and conductivity of soil in an agricultural context. The operating frequency of 20 MHz is selected so measurements are not influenced by surface polarization effects. By making the further assumption that the soil density and type is constant (reasonable in an agricultural measurement context), an in situ calibration can be performed that permits determination of the moisture content and conductivity using only a single frequency. As with the Sovik device, the Hilhorst device cannot simultaneously determine material density, and moisture content independent of conductivity and material type and particle size/shape effects.
In addition to the above-described deficiencies, all of the above-described devices exhibit inaccuracies due to a number of other factors such as the sensor used and the compaction process used to compact the material.
Another application is disclosed in Siconolfi, U.S. Pat. No. 6,125,297, in which an apparatus is described that determines the total body water content of living tissue using impedance spectroscopy using an electrical model of the body tissue. The device measures the complex impedance spectrum. As in Hilhorst, however, only moisture content and conductivity are calculated. Mean density effects are removed from the measurement by calibration. The device is also inaccurate because it is affected by the physiological state of the subject and by individual compositional variations.
In view of the foregoing, there is a need in the art for a material analyzer system that can accurately measure engineering properties such as density and moisture content of all varieties of materials.